Electric machines such as, for example, motors and generators may be used to generate mechanical power in response to an electrical input or to generate electrical power in response to a mechanical input. Magnetic, resistive, and mechanical losses within the motors and generators during mechanical and electrical power generation can cause a build up of heat. One of the limitations on the power output of the electric machines may be a temperature within the electric machines. If the temperature within the electric machine exceeds an operating capacity of the machine, malfunction or even damage of the machine can occur.
One method of limiting malfunction and damage of an electric machine includes determining a temperature of the electric machine and altering operation of the machine in response to the determination. For example, U.S. Pat. No. 3,629,676 (the '676 patent) to Ephraim, Jr. et al. discloses a system for controlling the power output of a locomotive generator as a function of traction motor temperature. The system utilizes an electrical heater element connected in series with a traction motor. The heater generates an amount of heat, which is a function of the amount of current being supplied to the traction motor such that the operating temperature of the traction motor is simulated. A temperature sensitive resistance element senses the simulated temperature and is connected with a control circuit to vary the excitation and output power of the generator as a function of the sensed temperature. As the simulated temperature increases past a maximum temperature, excitation of the generator is limited to reduce the current to the traction motor and, thereby, the temperature of the traction motor.
Although the system of the '676 patent may adequately control motor temperature in a locomotive application, it may be problematic. In particular, because only motor temperature is simulated and sensed, other temperature sensitive components within the same system may be unprotected. Further, because the temperature is simulated rather than directly measured, there may be situations in which the simulated temperature deviates from actual motor temperature, thereby allowing the system to be unnecessarily limited or, conversely, insufficiently protected. In addition, because only generator output is limited in response to excessive simulated temperatures, the response of the system may be slow and, if power storage is available, the traction motors may continue to operate at excessive temperatures until the storage is depleted. This extended operation at elevated temperatures may cause malfunction and damage to the traction motors.
The disclosed electric drivetrain is directed to overcoming one or more of the problems set forth above.